In the synthetic rubber industry, extensive use has been made of screw presses with intermittent thread flights in both the dewatering and final drying stages of manufacture. Zies U.S. Pat. No. 3,222,797 discloses such a drying press or "expander" in which the dewatered polymer fed to the press barrel inlet is initially propelled forwardly by a continuous flight unit of several turns or wraps, then advanced through a series of separate flights, and, finally, is ejected through a perforated die plate exit restriction. Rows of "breaker" bolts project radially into the cylindrical barrel between the individual thread flights to increase the resistance to rotation of the feed stock plugs as they are advanced through the casing by rotation of the feed worm. The die head restriction is variable so that the polymer will be compacted throughout a substantial portion of the barrel by the action of the pitched flights. Once the polymer is compacted and "worked," i.e., agitated and kneaded by screw action, heat is developed by friction and hysteresis effects and under the influence of external heating means. The temperature and pressure of the polymer and contained water rise during the process. As the polymer exits through the discharge dies, the pressure is abruptly reduced to atmospheric and the superheated water is released as steam as the polymer expands. This is how drying is accomplished.
Since there are a variety of polymer types with different properties, variable temperature and pressure requirements necessary for drying are usually achieved by proper selection of the size, shape, and number of discharge die openings.
When maximum throughput rates are desired, the die considerations must be adjusted to cause sufficient restriction and back pressure to produce compacting of the rubber through most of the length of the barrel, so that the polymer will receive maximum work input and heating. This often results in extreme pressures at the die head, e.g., as much as 1500 p.s.i.g., and extreme velocities of polymer through the dies. This can produce intolerable conditions at the expander exit because the violent explosions and high velocities produce a very fine product which sticks to the equipment and conveyors, is drawn into the ventilation system, and conveys poorly.
Another disadvantage in the described arrangement is in the tendency of the highly pressured and heated polymer to produce non-uniform crumb with stranded, dense product from some holes and fragmented, blowing fines from other holes.
In the above described prior art drying practice, all thread flights of the feed worm have a drive shaft wrap of approximately 95%. However, Zies U.S. Pat. No. 3,034,424 discloses a feed screw press for removing oil or the like from organic materials, while preventing damaging pressuring and heating of the liquid. Separate flights of the feed screw are variously shortened and relatively positioned to produce tortuous relief passages for returning excessively pressured product to the barrel inlet, to prevent excessive pressure and temperature build-up in the barrel.